The present invention relates to integrated circuits, and more particularly, to voltage generators which provide different reference voltages required for supplying integrated circuits.
External power supplies for integrated circuits now vary between three volts and ten volts, whereas the voltages required by the internal power supplies for the electrical circuits within the integrated circuits are, depending on the application, 2.5 volts, 3 volts, 5 volts and 7 volts. These voltages are within xc2x110%. It is therefore imperative that an integrated circuit itself generate these different voltages in order that they be independent of the power supply voltage and of temperature. For instance, the temperature may vary between xe2x88x9240xc2x0 C. and 125xc2x0 C.
To this end, there has been proposed a regulated voltage generator which exploits the properties of a reference voltage given by a circuit described in an article by E. Vittoz and J. Fellrath, entitled xe2x80x9cCMOS Analog Integrated Circuits Based on Weak Inversion Operationxe2x80x9d, published in IEEE Journal of Solid State Circuits, Vol. SC-12, no. 3, 1997, pages 224-231. This voltage reference circuit is generally known as a bandgap voltage reference circuit.
This prior art circuit supplies a reference voltage of 1.28 volts, known as the bandgap voltage, which is constant over a wide range of supply voltages and temperatures. To obtain the different required voltages, the circuit""s output voltage is applied to gain stages, with each gain stage producing one of the required voltages.
However, these gain stages are sensitive to the supply voltage and to temperature, and the same holds for the power stage that follows them for supplying the required power. As a result, the voltages supplied vary significantly as a function of power supply voltage and of the temperature.
An object of the present invention is to provide a generator for at least one regulated voltage that is not very sensitive to variations over a wide range of power supply voltages and temperatures.
This object is achieved by using a potential barrier reference voltage circuit, known as a bandgap type of circuit, and at least one gain stage. To provide a regulated voltage generator that is not sensitive to variations in the power supply voltage and temperature, the characteristics of the reference voltage are degraded to compensate for the variations due to the gain stage. The reference voltage then delivers a voltage which is a function of temperature variations opposite to that of the gain stage.
Another object of the present invention is to provide a generator producing a plurality of regulated voltages by implementing several gain stages.
The invention thus relates to a regulated voltage generator for supplying at least one regulated voltage to an integrated circuit comprising a bandgap type of reference voltage circuit and at least one gain stage. The bandgap type of reference voltage circuit comprises a current generator which supplies a bipolar transistor configured as a diode via a load resistor connected to the emitter of the bipolar transistor.
The gain stage comprises two MOS transistors in series between the supply voltage and a ground potential. The gate of a first transistor is connected to the gate of the output transistor of the current generator, and the gate of the second transistor is connected to the output of the bandgap type reference voltage circuit.
The characteristics of the first and second transistors are chosen to obtain the regulated voltage. The value of the load resistor is chosen such that the emitter-base voltage of the bipolar transistor varies with temperature in a manner to compensate for the variation of the gate-source voltage of the second transistor as a function of temperature.